1. Field of the Invention
The present invention relates to an image forming apparatus such as a copying machine, an electrophotographic printer and the like, and more particularly, it relates to an image forming apparatus having a two-sided copying function or multi-copying function.
2. Related Background Art
Recently, the multi-functioning of image forming apparatuses has progressed, and image forming apparatuses having a two-sided copying function or multi-copying function have been popularized. For example, when images are formed on a single sheet on both sides, images are formed on a predetermined number of sheets on their one sides, which sheets are, in turn, stored in an intermediate tray. Then, the sheets stored in the intermediate tray are separated one by one and the separated sheet is re-fed to an image forming station where an image is formed on the sheet on the other side.
However, in re-feeding the sheet, the image formed on the sheet rubs against the other sheet to smudge an end portion of the other sheet. Particularly, in a case of an arrangement wherein the sheets are successively re-fed from a lowermost sheet of the sheet stack stored on the intermediate tray, the speed-up of the operation is contemplated so that the re-feeding of the sheet can be effected before the predetermined number of sheets are received in the intermediate tray. However, in this case, since the lowermost sheet is fed first, the sheet stack on the sheet to be separated is always entered into a separating station Z where the lowermost sheet is separated from the next sheet on the lowermost sheet while being rubbed against the next sheet (FIG. 8). As a result, since the sheets are stacked or stored on the intermediate tray with the imaged surfaces facing up in the two-sided copying mode, toner particles 52 forming the image on the sheet to be fed by a re-feeding means are moved while being rubbed against the undersurface of the upper sheet 53 at a range 52a (shown by the hatched area), thus causing the contamination 54 at the end portion of the upper sheet.
Further, in a case where a mold-releasing agent or parting agent is coated to a fixing roller to prevent the offset phenomenon, since the parting agent is absorbed in the sheets, the coefficient of friction between the sheets is decreased. As a result, it will be difficult to cause the above-mentioned contamination of the sheet in the re-feeding operation. However, as shown in FIG. 9, since a length A of an area of the fixing roller 61 to which oil is coated to the fixing roller by means of an oil coating means 62 is greater than a width B of the largest sheet S which can be used in an image forming apparatus, the consumption of oil will be increased.